1. Field of the Invention
The present invention relates to a master device capable of connecting to slave devices, which correspond to a plurality of motors, via a serial communication bus for carrying out data transmission and reception, in order to control a system in which motors of the same number as inverters can be connected in parallel to a DC link part, which is connected to an alternating-current power source via a converter, via each of the inverters.
2. Description of Related Art
A master device that is connected to a slave device via a serial communication bus for carrying out data transmission and reception in order to control a motor in a system, which includes motors for driving a feed axis and a main axis of a machine tool, an industrial robot arm, etc., is proposed in, for example, Japanese Unexamined Patent Publication (Kokai) No. 2008-242728 (JP2008-242728A).
In such a system which includes the motors, the master device is comprised of a converter control device configured to control a converter in order to convert alternating-current power supplied from an alternating-current power source into direct-current power, an upper control device such as a CNC (computer numerical control), etc., and the slave device is implemented by an inverter control device configured to control an inverter in order to convert the direct-current power which is converted by a converter into alternating-current power, etc. For example, in the case where the master device is the converter control device and the slave device is the inverter control device, data such as motor status information, alarm information, etc., is shared between the converter control device and the inverter control device by carrying out data transmission and reception via a serial communication bus. In the case where the master device is the upper control device and the slave device is the inverter control device, data such as motor position or speed command data, etc., is transmitted from the upper control device to the inverter control device via the serial communication bus and data such as current value data, motor position or speed data, etc., is transmitted from the inverter control device to the upper control device via the serial communication bus, thereby the data is shared between the converter control device and the inverter control device.
A serial communication among a plurality of devices can transmit and receive an amount of data larger than the amount of data that can be transmitted and received by parallel communication with electric lines in a number smaller than the number of electric lines used for the parallel communication (for example, two electric lines are used in the case of RS422 serial communication).
In the serial communication, if it is desired to increase the amount of data to be transmitted and received per unit time, and therefore it is necessary to increase the data communication speed, i.e., extend the bandwidth of a signal necessary to transmit data. In the case where the bandwidth of the signal necessary to transmit data is extended, it is also necessary to extend the bandwidth of a filter provided in at least one of the master device and the slave device in order to remove noise received in the serial communication. Therefore, as the bandwidth of the filter is extended, the influence of noise received in the serial communication increases.
Measures against the noise received in the serial communication are taken in order to extend the bandwidth of the signal necessary to transmit data and to maintain the serial communication with high reliability. As measures against the noise received in the serial communication, there are an increase in the thickness of the cable used for the serial communication in accordance with an increase of the data communication speed in order to reduce an impedance of the cable used for the serial communication, an increase in the thickness of the shield for the cable used for the serial communication in accordance with an increase in the data communication speed in order to prevent fluctuations in the electric potential due to an external influence, a reduction of the length of the cable used for the serial communication in accordance with an increase in the data communication speed in order to reduce the impedance of the cable used for the serial communication, etc.
Consequently, the cost necessary to take such measures against the noise received in the serial communication and restrictions on the length of the cable used for the serial communication increase as the data communication speed increases.
In the above-mentioned system which includes the motors, in order to keep the period necessary to periodically carry out necessary data transmission and reception between the master device and a plurality of the slave devices via the serial communication bus during the period from the start of the drive of the motor to the stop of the drive of the motor (data transmission and reception period) at a fixed time (for example, 1 msec) or less, it is necessary to increase the data communication speed of data transmitted and received between the master device and a plurality of the slave devices as the number of slave devices, which correspond to the motors actually connected to the DC link part, increases.
In the above-mentioned conventional system which includes the motors, in order to keep the above-mentioned data transmission and reception period at a fixed time or less, the communication speed of data transmitted and received between the master device and a plurality of the slave devices via the serial communication bus is set to a fixed communication speed regardless of the number of slave devices, which correspond to the motors actually connected to the DC link part.
In the case where the communication speed of data transmitted and received between the master device and a plurality of the slave devices via the serial communication bus is set to a speed more than necessary, the cost necessary to take measures against the noise received in the serial communication and restrictions on the length of the cable used for the serial communication increase more than necessary.
For example, in a system in which the number of slave devices, which correspond to the maximum number of motors that can be connected to the DC link part, is ten, a case where the data communication speed is set to 500 kHz in order to keep the above-mentioned data transmission and reception period for carrying out data transmission and reception between the master device and ten slave devices at 1 msec, is envisaged.
In this case, even when the number of slave devices, which correspond to the motors in a number less than the maximum number (ten, in this case) actually connected to the DC link part, is five, the data communication speed is 500 kHz, and therefore, the above-mentioned data transmission and reception period when the number of slave devices is five is 0.5 msec. That is, in order to keep the data transmission and reception period at 1 msec when the number of slave devices is five, it is sufficient to set the data communication speed to 250 kHz.
Consequently, the communication speed is higher than the data communication speed necessary to keep the data transmission and reception speed at 1 msec, and therefore, the cost necessary to take such measures against the noise received in serial communication and the restrictions on the length of the cable used for the serial communication increase more than necessary.
On the other hand, in the case where the number (for example, five) of slave devices, which correspond to the motors in a number less than the maximum number (in this case, ten) actually connected to the DC link part, is supposed and the data communication speed is set to low so that the cost necessary to take measures against the noise received in serial communication and the restrictions on the length of the cable used for the serial communication do not increase, it is no longer possible to maintain the necessary data transmission and reception period.
For example, in a system in which the number of slave devices, which correspond to the maximum number of motors that can be connected to the DC link part, is ten, the number of slave devices, which correspond to the motors in a number less than the maximum number (in this case, ten) actually connected to the DC link part, is supposed to be five, and the case where the data communication speed is set to 500 kHz in order to keep the above-mentioned data transmission and reception period for carrying out the data transmission and reception between the master device and the five slave devices at 1 msec.
In this case, even when the number of slave devices, which correspond to the maximum number (in this case, ten) of motors actually connected to the DC link part, is ten, the data communication speed is 500 kHz, and therefore, the above-mentioned data transmission and reception period when the number of slave devices is ten will be 2 msec. As described above, in the case where the slave devices in a number (in this case, ten which is the maximum number) exceeding the number of slave devices used, it is no longer possible to maintain the necessary data transmission reception period (1 msec).